The present invention relates to the use of charge coupled devices (CCD's) for video signal collection, and more particularly to an apparatus and method for applying a correction to compensate for low-pass filtering effects which may occur in charge transfer devices.
Charge coupled devices (CCD's) are used commonly in video imaging. In the simplest form, a linear array of photosensors is used to form charge packets proportional to light intensity incident on each photosensor, and these packets are shifted to a detector point for readout. CCD's are used for dynamic storage and withdrawal of charge and may be modeled as a series of field effect transistors (FET's). When a gate pulse is applied to one of these FET's, nearby charge carriers in a p-type semiconductor underneath the gate are repelled, creating a depletion region in the semiconductor. This depletion region creates a potential well which is used to store charge.
In a typical CCD imaging array, multiple linear CCD imaging arrays are arranged in a matrix configuration such that the readout stage of each linear array is coupled to a respectively different stage of a CCD shift register, which operates at a relatively high clock rate. A line of charge packets, one packet from each linear imaging array, is simultaneously shifted into the respective stages of the CCD shift register. These charge packets are then shifted out of the shift register at relatively high speed. In this configuration, each charge packet corresponds to an image pixel and each line of packets loaded in parallel into the CCD shift register corresponds to the active video portion of a scanned line of a video image.
Typically, a CCD shift register (CCD SR) comprises a row or array of FET's. A means is provided to allow charge to flow from one potential well to an adjacent one quickly without loss of much charge. Voltages are applied to the FET's non-uniformly so that the potential wells vary with time and location within the CCD line or array. The voltages are clocked to cause the potential well to decrease in one FET at the same time it is increasing in the adjacent FET, causing the charge to shift. Charges can thus be applied, moved and collected.
One of the problems inherent in the charge transfer process is that some of the charge is retained by each cell as the charge packet transfers along the CCD SR. When a residual charge remains in a potential well, it is added to the next charge packet which is transferred into the well. This adding of residual charge averages the charges over as many wells as are transferred. In a CCD imaging device, the video signal for one picture element (pixel) will be averaged with the values in other pixels, causing the picture at the far end of the imaging matrix (i.e., away from the output stage of the CCD SR) to appear softened, or low-pass filtered.
FIG. 1 shows three adjacent elements from a conventional linear CCD SR in which residual charges occur. A line of charges is loaded into the CCD SR elements simultaneously, so there are no residual charges the beginning of the line. The second element 2 receives an input signal X, which is the charge transferred from the first element 1 to second element 2. Second element 2 provides an output signal Y which is the charge transferred to the third element 3. When a charge is transferred out of element 2, a fraction of the charge designated .alpha. is retained in element 2, and the remaining fraction (1-.alpha.) is transferred.
This incomplete transfer of charge makes the CCD element behave like a filter with feedback. At any given time i, the charge transferred by element 2 is equal to the sum of fraction of the input signal provided by element 1 which is transferred at time i, plus the fraction of the last prior charge in element 2 which was retained at time i-1. This is expressed in equation (1). EQU Y.sub.i =.alpha.Y.sub.i-1 +(1-.alpha.)X.sub.i ( 1)
Equation (1) is a linear difference equation which defines a recursive sampled data filter, also known as an infinite impulse response filter. The element 2 behaves like a low pass filter with feedback. The infinite impulse response designation signifies that even when no charge is injected into an element, it will still have an output signal due to the feedback effect of the residual charge.
The inventors have determined that the value of .alpha. tends to increase as the time between charge transfers decreases. Thus, the picture softening problem is more acute for high speed devices, such as high definition television (HDTV) cameras, than for cameras used, for example, to produce NTSC images.
FIG. 2 shows a block diagram of the behavior of a single CCD element. The value of input signal X is the charge stored in circuit 10. There is an effective feedback loop 12 with a gain 14 of .alpha. and a delay 16 of one sample period T.
FIG. 3 shows a two dimensional CCD array 20. CCD array 20 comprises m rows 22a-m, each row having n charge transfer elements 24a-n per row. Each charge transfer element 24 in the last row of the array 20 provides an output signal 30a-m to a respectively different storage element 26a-n of CCD shift register 26.
FIGS. 4a-e show how the charges are initially injected into each element of shift register 26a-n from the bottom CCD row 22m, and subsequently shifted to the right within shift register 26. In a conventional CCD system, the shift register elements 26a-n have all transferred their charges before a new row of charges is injected into elements 26a-n in parallel. As shown in FIG. 4a, the initial charges injected into shift register elements 26a-n, are (from right to left) N, M, L, K, J. As shown in FIG. 4b, after one period, all of the charges are shifted to the right. Because of the residual charge, the rightmost shift register element 26n will have a charge equal to .alpha.N+(1-.alpha.)M. As shown in FIG. 4c, after the second period, element 26n will have a charge equal to .alpha..sup.2 N+.alpha.M+(1-.alpha.).sup.2 L. For brevity, the higher order terms (.alpha..sup.2, .alpha..sup.3, etc.) are omitted from FIGS. 4c-e. FIGS. 4d and 4e similarly show the charge in each element after 3 and 4 periods, respectively. After the n.sup.th period, the charge in element 26n will be given by equation (2). ##EQU1## where W=the original charge transferred from the CCD array
Y=the charge read out of the shift register
As n becomes larger (i.e., as the leftmost values are read from the shift register), more sample values are mixed in with the value being read. The result is that the error due to averaging is greater for the pixels which are read later (those originally on the left side of the array). This causes the picture to be softened (low pass filtered) more on the left side than on the right.
In Ohbo, M. et al., "A New Noise Suppression Method for High Definition CCD Cameras" a Reflected Delayed Signal (RDS) method for compensating for the low pass filter effect is discussed. Ohbo provides the modulated CCD output signal to a delay circuit. In the delay circuit, the output signal is delayed by half of the CCD output signal period and this delayed signal is then subtracted from the CCD output signal. This method is used to recover the CCD signal from the multiphase signal provided directly by the CCD. This signal may be analogized to a sampling signal modulated by the image data. This circuit substantially cancels low frequency noise in the CCD output signal. It does not, however, affect the frequency content of the baseband video signal which may be obtained from the CCD output signal using conventional sample-and-hold techniques.
U.S. Pat. No. 4,435,730 to Bendell et al. discusses an apparatus for improving CCD signal to noise characteristics. The apparatus processes two signals from a CCD, the reset drain and the floating diffusion signals. The drain has poor signal to noise characteristics at high frequencies, and the diffusion has poor signal to noise characteristics at low frequencies. The drain signal is filtered using a low pass filter (LPF), and the diffusion signal is filtered using a high pass filter (HPF). The LPF and HPF have complementary transfer characteristics. Their transient responses are opposite in phase, so that when the output of the LPF and HPF are combined, the transient characteristics cancel.
Chamberlain, High Speed, Low Noise, Fine Resolution TDI CCD Imagers, 1242 SPIE 252 (1990) discusses another method in which modifications to the CCD array are made. Time delay integration (TDI) is a technique in which a column of CCD elements are sequentially exposed to the object being imaged, such that the clock rate for shifting charges is synchronized with the velocity of the object. As the signal moves down the column, it grows. For a column with n TDI stages, (i.e., having n elements) the signal at the readout is n times the signal without TDI, but the noise is only increased by n.sup.1/2.
CCD's with TDI architectures employ a buffer output video amplifier at the end of each CCD readout shift register. Chamberlain adds a two stage source follower amplifier. This amplifier is formed using special low doped source drain buried channel MOSFET devices. The goal of this technique is to further reduce the value of .alpha., the residual charge fraction, and a value of 10.sup.-5 is quoted.
TDI techniques are not without cost, however. For an array of given size, the dynamic range of the imager decreases as the number of TDI stages increases.